Oxygen '96
Early Stages of Oxygen Precipitation in Silicon
UNIAXIAL STRESS MEASUREMENTS ON OXYGEN-IMPLANTED SILICON: THE
889 cm-1 LOCAL MODE
B. Bech Neilsen T. Lisby and K. Bonde Nielsen.
Institute of Physics and Astronomy, University of Aarhus, DK-8000
Aarhus C. Denmark
Local vibrational mode spectroscopy has been applied previously to
study the early stages of oxygen agglomeration in crystalline silicon.
In particular, an oxygen local mode at 889 cm-1 has been
observed in electron-irradiated Czochralski-grown silicon annealed at
about 300 degrees C. Experimental evidence suggests that this mode
originates from a defect containing two weakly coupled oxygen atoms.
The defect has tentatively been identified with VO2, i.e.
a monovacancy in which two oxygen atoms saturate the four dangling
bonds. This model is also consistent with recent ab initio
theoretical calculations. However, no experimental information on the
symmetry of the defect has been presented so far and the
microstructure remains unidentified.
In this work, the 889 cm-1 centre is studied by uniaxial
stress spectroscopy. Silicon crystals with dimensions 2x2x10
mm3 were cut from float-zone material. The crystals were
aligned with the [001], [110] or [111] axis and two opposing 2x10
mm2 sides were polished. Oxygen ions (O5+) were
implanted at room temperature at 168 different and equidistant
energies in the range from 12 to 33 MeV. The dose at each energy was
chosen to yield a nearly uniform oxygen profile extending from 8.0 to
22.2 micro-m below the surface with a local concentration of
1.5x1019 cm-3. Just after the implantation, the
local modes of the VO defect (A-centre) at 836 cm-1 and of
interstitial oxygen at 1136 cm-1 are observed with infrared
absorption spectroscopy at 8 K. After a 30-min. anneal at 330 degrees
C, several new oxygen-related modes appear as reported previously.
Among these, there is a mode at 889 cm-1 which shifts up in
frequency to 895 cm-1 when the temperature is lowered to 8
K. The uniaxial stress measurements show that the defect responsible
for this mode either has tetragonal (e.g. D2d) symmetry and
that the mode is a two dimensional E mode or that the centre has
orthorhombic I (C2v) symmetry and that the mode is
one-dimensional. These findings are consistent with the
VO2 model which has D2d symmetry but other
structures with C2v symmetry cannot be ruled out. If the
VO2 structure is correct, then the motions of the two
oxygen atoms corresponding to the 889 cm-1 mode are
uncoupled, as predicted by theory.
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Last modified: Mon Feb 19 17:20:04 GMT 1996
JG
